As the oscillation wavelength of a semiconductor laser device incorporated into an optical pickup used for reproduction and recording of an optical disk is shortened, the recording density of the optical disk is increased. Thus, nitride semiconductor laser devices which oscillate at wavelengths ranging from blue to violet have been developed, and optical pickups using such nitride semiconductor laser devices have been applied to practical use. Also, nitride semiconductor laser devices that oscillate in the ultraviolet region may be applied to solid-state lighting systems that excite fluorescent material by ultraviolet light, and these solid-state lighting systems are expected to replace fluorescent lamps. On the other hand, light-emitting diodes made of nitride semiconductors are used as blue or white light emitting diodes.
Looking back on the history of the development of nitride semiconductor light-emitting diodes and nitride semiconductor laser devices, there was a major challenge in that it was difficult to obtain low-resistance p-type crystals for nitride semiconductors. It was known that nitride semiconductors, for instance gallium nitride (GaN), were semiconductor materials having numerous lattice defects and, when not doped with a dopant, exhibited n-type conductivity because of nitrogen holes produced in the crystals. Furthermore, even if a nitride semiconductor was doped with a p-type dopant, the nitride semiconductor only became high-resistance i-type, and it was thus difficult to obtain low-resistance p-type crystals.
To address this, a technique was developed in which a p-type semiconductor is obtained by heat-treating a nitride semiconductor doped with a p-type dopant (see Patent Document 1, for example). This technique is based on the assumption that hydrogen (H) present in the semiconductor binds to magnesium (Mg) serving as the p-type dopant and thus prevents the Mg from functioning as an acceptor, resulting in high resistance. In this technique, therefore, gallium nitride (GaN) doped with Mg is heat-treated to remove hydrogen (H) and hence allow the Mg to function properly as an acceptor, thereby obtaining low-resistance p-type gallium nitride. Since the publication of this technique, various research institutes have conducted research on activation heat-treatment techniques for obtaining p-type nitride semiconductors.
Nevertheless, even with those activation heat-treatment techniques, it was still difficult to completely remove hydrogen (H) from a p-type nitride semiconductor. Therefore, a technique was developed in which residual hydrogen remaining in a nitride semiconductor layer is attracted by a metal hydride layer, thereby promoting activation of magnesium (Mg) (see Patent Document 2, for example). This technique particularly aims to increase the carrier concentration in a p-type contact layer that forms an ohmic electrode, and adopts a configuration in which a metal hydride layer is interposed between the p-type contact layer made of a nitride semiconductor and an electrode. Since this metal hydride layer promotes the activation of Mg and permits the contact layer to have a sufficiently high carrier concentration, an ohmic contact whose contact resistance is very low is achievable. However, the inventor of Patent Document 2 himself mentioned that the ohmic contact formed with this technique had a problem in stability over time. And to overcome this problem, the inventor disclosed a technique in which a hydrogen absorber metal is deposited and then removed and an electrode is formed again (see Patent Document 3, for example).
In the foregoing, the background art for the p-type nitride semiconductor formation method and the reliability of a p-type nitride semiconductor used as a contact layer have been described. Next, background art for reliability testing on a nitride semiconductor laser device will be described. A life test was conducted on a nitride semiconductor laser device, and the results of a study of a deterioration mechanism in the nitride semiconductor laser device have been reported (see Non-Patent Document 1, for example). This nitride semiconductor laser device has a configuration shown in FIG. 11.
As shown in FIG. 11, a buffer layer 1102 made of n-type GaN is grown on a sapphire substrate 1101 using an ELO (epitaxial lateral overgrowth) technique. Subsequently, a cladding layer 1103 made of n-type AlGaN, a guide layer 1104 made of n-type GaN, a multi-quantum well active layer 1105, an overflow suppression layer 1106 made of p-type AlGaN, a guide layer 1107 made of p-type GaN, a superlattice cladding layer 1108 made of p-type AlGaN/GaN, and a contact layer 1109 made of p-type GaN are stacked in this order on the buffer layer 1102, thereby obtaining a laser structure.
The contact layer 1109 and the superlattice cladding layer 1108 are patterned by etching so as to have a striped ridge structure extending in directions coming out of and into the drawing. On the ridge structure, an insulating film 1110 having an opening, and a p-side electrode 1111 composed of a Pd/Pt/Au multilayer film are formed. An n-side electrode 1112 composed of a Ti/Pt/Au multilayer film is also formed on part of the buffer layer 1102 exposed by etching.
In Non-Patent Document 1, dislocation density in a long-life laser device is compared with that in a short-life laser device. The comparison results show that dislocation increase from through-dislocations, or structural change in through-dislocations, each well known as a deterioration mechanism in a gallium-arsenide(GaAs)-based or indium-phosphide(InP)-based laser device, was not observed. Also, since the rate of deterioration was proportional to the square root of the aging time, it was assumed that the deterioration developed through a diffusion process. And the conclusion was made that the deterioration was caused by the diffusion of defects to the active layer along dislocations.    Patent Document 1: Japanese Laid-Open Publication No. 5-183189    Patent Document 2: Japanese Laid-Open Publication No. 8-32115    Patent Document 3: Japanese Laid-Open Publication No. 2002-75910    Patent Document 4: Japanese Laid-Open Publication No. 2004-320024    Non-Patent Document 1: S. Tomiya et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 10, No 6, pp 1277-1286, November/December 2004